Sound reproducing apparatus



April 5, 1938. R. s. MORSE ET AL SOUND REPRODUCING APPARATUS 2 Sheets-Sheet 1 AMPLIFIER Filed NOV. 6, 1936 coumoummcn rouf CONTROL AUDIO L INPUT 1 iilO U msmmmuqlgg m ATTORNEYS April 5, 1938.

R. S. MORSE ET AL SOUND REPRODUCING APPARATUS 2 Sheets-Sheet 2 Filed Nov. 6, 1956 msmmawwwod INVENTORS BY M ATTORNEYS.

Patented Apr. 1938 UNITED STATES PATENT OFFICE SOUND REPRODUCIN G APPARATUS New Jersey Application November 6, 1936, Serial No. 109,557

5 Claims.

This inventionrelates to sound reproducing apparatus. More particularly it relates to electrical tone control devices for use in sound reproducing apparatus.

5 In reproducing sound in radio receiving sets or sound motion picture projectors, it is often desirable to attenuate the higher frequencies, especially to eliminate unwanted noises arising from static or, in the case of sound motion picture projectors, from old or particularly noisy optical sound records. If the higher frequencies are omitted during the reproduction of sound, a much more pleasing effect to the ear and greater speech intelligibility will be obtained if the lower frequencies are also attenuated to some extent. Many methods of attenuating the higher frequencies are known and arrangements have been provided for eliminating the lower frequencies such as 60 cycle hum and the ground noises in sound records. i

It is an object of the present invention to provide means for attenuating both the higher frequencies and the lower frequencies. f

Another object of the invention is to provide an arrangement for attenuating both the higher and lower frequencies simultaneously and in a predetermined relation.

It is a further object of this invention to provide an arrangement for doing this which is particularly suitable for use on a sound motion picture projector. It is to be understood, however, that the invention may similarly be applied to other electrical circuits wherein audio frequency impulses are being transmitted, such as 35 in radio broadcasting or in sound recording.

The terms high, higher, low and lower as applied to audible frequencies refer as usual to the two extreme portions of the audible range. The limits of these portions are not exactly definable but for convenience, we adopt the arbitrary custom of considering the normal frequency range as from about 400 to 3500 cycles per second. Our invention generally affects a portion of the "low" region below 400 cycles and 01' the high" region above 3500 cycles, but it may also encroach slightly on the "normal range as thus arbitrarily defined.

Other objects and advantages of this invention 50 will be apparent from the following description when read in connection with the accompanying drawings in which:

Fig. 1 is a schematic drawing to indicate the general arrangement of this invention. Iflg. 2 indicates schematically how this invention is used in connection with sound motion pictures.

Figs. 3, 4, 5, and 6 are skeleton circuit diagrams showing various forms which this tone contrci invention may take. 5

In Figure 1 an electrical current carrying the audio frequency impulses is received from a source H and fed into an electrical circuit including a control amplifier H, a tone control circuit l3, further amplification stages l4 and a loud speaker 15 which translates the impulses into sound. The higher frequencies and/or the lower frequencies present in the audio intelligence received from the source H may be attenuated by the tone control circuit it.

In Figure 2, a motion picture film l6 bearing .a sound track I! is moved in a well-known manner past a projection lamp i8 and a projection objective IQ of any suitable motion picture projector and through a sound head, so that the sound track I? intercepts the light beam from an exciter lamp 20 anda modulated light beam produced thereby impinges on a photoelectric cell 2| ,in the usual way. The output of the photoelectric cell 2| with its accompanying electrical circuit I2 is introduced to a tone control circuit I3 which in accordance with the invention includes a high' frequency attenuating means 22 and a low frequency attenuating means 23 which preferably are coupled so as to be con- 0 trollable simultaneously. An amplifier l4 and a loud speaker I5 complete the reproducing system.

Similar reference numerals refer to similar units in all of the figures. In Figure 3, an audio input source ll supplies audio impulses to the control grid of space discharge tube which preferably is a tetrode and which together with its accompanying circuit forms a control amplifier 12 of a well-known type. The output of this unit' is supplied through a condenser 24 to the grid of another space discharge tube shown as a triode 26. A grid leak resistance 25 is placed in the grid circuit of the triode 26 iii the usual way. In parallel with this grid leak resistance 25 and serving as a by-pass for a portion of the higher frequencies present, a variable condenser 21 and a variable resistance 28 are arranged in series so that an increase of the capacity 21 or a decrease of the resistance 28 attenuates the higher frequencies. In accordance with the invention, means are provided for also attenuating the lower frequencies and as shown in Figure 3, this means may comprise a variable condenser 29 and a variable resistance 30 connected in series between thoeothodooi'thotubellandnomdmdinu condensers 21 and 29 are arranged for joint control, as indicated by broken lines joining the conventional variability arrows. These coupling arrangements assure that an adjustment to increase the resistance 28 automatically decreases the resistance 30 and vice versa and also that increasing the capacity 21 automatically decreases the capacity 29 and vice versa.

In order to attain the object of the invention, it is sufficient to have either the resistances or the capacities variable, but both may be variable, as shown, if desired. Thus the operation of the invention provides attenuation of the lower frequencies in predetermined relation to the attenuation of the higher frequencies.

In Figure 4, the output of the control amplifier unit I2 is supplied to the grid of an amplifier tube 26' primarily through a condenser 3|. In parallel with this condenser 3| is another condenser 32 and a variable resistance 33 in series. The total coupling capacity between the unit l2 and the grid of the tube 26 is thus the combined capacity of these parallel circuits. A decrease in this total coupling capacity, resulting from an increase in the resistance 33, will cause a reduction in relative intensity of the lower frequencies.

Attenuation of the higher frequencies is obtained in a well known manner, by by-passing these frequencies througha condenser 34 and a variable resistance 35 across the grid circuit of the tube 26. The resistances 33 and 35 are preferably arranged for jointcontrol so that adjustment which decreases the resistance 35 simultaneously increases the resistance 33. Thus, simultaneous attenuation of both the higher and lower frequencies is accomplished.

Figure shows a circuit arrangement similar to Figure 4, but in this case, the capacity of the condenser 3| is adapted to be augmented by the capacity of condensers 32 and 36 shunted across it in series when a two way switch 31 is in one of its alternative positions (position A). Moving this switch 31 to its other position B places the condenser 32 in series with a condenser 34 so as to by-pass and thus attenuate the higher frequencies. Since with the switch 3'! in position 3, the condenser 3| is not augmented by the capacity of the condensers 36 and 32 in series, the lower frequencies are also attenuated. In this embodiment of the invention, the amount of attenuation is not continuously controllable, but by a certain choice of the capacities of the condensers 3|, 32, 35, and 36, a desirable arrangement is produced wherein normal reproduction of all frequencies is obtained when the switch 31 is in position A and. a desirable amount of high frequency attenuation together with the amount of low frequency attenuation which will retain the best tone balance is obtained when theswitch 31 is in position B. y

Figure 6 shows an arrangement which is similar to either that in Figure 4 or that in Figure 5. In this case, the switch 31 is replaced by a variable resistance 36 so that continuous control of the simultaneous attenuation of the higher and lower frequencies is obtainable.

The above description is merely to illustrate a few of the forms which this invention may take and it may be applied to other suitable arrangements which include means for attenuating the higher frequencies such as those well known in the art. It is to be understood that the invention is not limited to the specific arrangements shown but is of the scope of the appended claims.

What we claim and wish to secure by Letters Patent of the United States is:

I. In apparatus of the character described, a vacuum tube amplifier, an input circuit therefor including adjustable means for attenuating certain low frequency bands, adjustable means for attenuating certain high frequency bands, and means for jointly controlling the adjustment of both of saidmeans in the same direction, whereby the low and high frequency bands are simultaneously attenuated in a predetermined relation.

2. In apparatus of the character described, an audio frequency coupling condenser and a tone control device comprising a shunting condenser to augment the capacity of said coupling condenser, a condenser to by-pass the higher frequencies transmitted by said coupling condenser and a two position switch arranged to connect alternatively said shunting condenser in parallel with said coupling condenser and the circuit of said by-pass condenser.

3. In an electrical circuit for transmitting audio frequency impulses, a coupling condenser adapted to transmit said impulses, a variable impedance including a condenser in series and shunting said coupling condenser, a second variable impedance adapted to by-pass the higher frequencies present among the impulses transmitted by said coupling condenser, and mechanical means for controlling the two variable impedances so that said impedances are variable simultaneously, inversely and in predetermined relation.

4. In an electrical circuit for transmitting sound intelligence, the method of improving the tone quality of audio frequency current having objectionable high frequencies foreign to said sound intelligence comprising variably attenuating the high, frequencies to reduce the relative intensity of said objectionable high frequencies and simultaneously and correspondingly variably attenuating the low frequencies to maintain good intelligibility.

5. In an electrical circuit for transmitting audio frequency impulses, a variable impedance including a condenser in series relation to the circuit for preferentially transmitting the normal and high frequencies, and attenuating the low frequencies, a second variable impedance including a condenser in shunt relation to the circuit for partially by-passing the high frequencies only and mechanical means connecting the impedances for simultaneously changing them oppositely in predetermined relative proportions whereby a decrease in the shunt impedance is accompanied by an increase in the series impedance.

WALTER K. GRIMIWOOD. RICHARD S. MORSE. 

